As a magnetic recording medium adapted to be mounted in a hard disk drive device (HDD device), there is a magnetic disk. The magnetic disk is produced by stacking a magnetic layer and a protective layer over a substrate such as a substrate in which a NiP film is coated on a metal plate made of an aluminum-magnesium alloy or the like, a glass substrate, or a ceramic substrate. Conventionally, aluminum alloy substrates have been widely used as substrates for magnetic disks. However, following the reduction in size and thickness and the increase in recording density of magnetic disks in recent years, there have been used glass substrates which are excellent in surface flatness and in strength even with small thickness as compared with the aluminum alloy substrates.
With respect to a magnetic disk produced by forming at least a magnetic layer on such a magnetic disk substrate, defects present on a surface thereof are inspected. For example, Patent Document 1 discloses that defect detection of a magnetic disk substrate is performed using a defect detection optical system which comprises a light projection system that projects a light beam having a width in a direction at a right angle to a main scanning direction to relatively scan a transmissive substrate, and further comprises a light receiving system comprising a high magnification objective lens that receives scattered light from the transmissive substrate in a direction perpendicular to a surface of the transmissive substrate, and further comprising a light receiving device having light receiving elements that are arranged along a direction of corresponding image formation in the right-angle direction and receive light from the objective lens, thereby forming an image at a scanned position of the transmissive substrate on the light receiving elements.